Electrolytic anode

BACKGROUND OF THE INVENTION
In the electrolysis of brines to yield chlorine or sodium chlorate, an anode and a cathode are provided within an electrolytic cell. An electrical potential is established between the anode and the cathode whereby the negatively charged chloride ions are attracted to the anode. At the anode, the reaction:
Cl.sup.-.fwdarw.Cl + e.sup.-
Occurs. Thereafter, the monoatomic chlorine atoms, i.e., nascent chlorine, combine to form diatomic chlorine molecules according to the reaction:
2Cl.fwdarw.Cl.sub.2
In chlorine production, the chlorine molecules form gas bubbles on the surface of the anode, and chlorine is subsequently recovered as a gas above the electrolyte. In chlorate production, the neutral to basic pH of the cell causes further oxidation of the chlorine molecules and ultimately results in the formation of chlorate ion, C10.sup.-.sub.3.
In both processes, the anode is subjected to rigorous conditions. For example, the anode is subjected to conditions of attack by nascent chlorine atoms in an acidic media under anodic conditions. This necessitates the use of a particularly corrosion-resistant material for the electrode. However, the electrode material must have electrocatalytic properties for the chlorine evolution reaction and sufficient electroconductivity to permit the passage of electrical current therethrough with a minimum IR voltage drop with an electrode coating thickness sufficient to protect the substrate from the effect of the electrolyte and electrolytical reaction.
SUMMARY OF THE INVENTION
It has now been found that these desirable properties may be provided by an electrode having a suitable electroconductive substrate and a layer of an electroconductive, intermetallic composition of a platinum group metal and a transition metal. The platinum group metals used to provide the intermetallic composition are ruthenium, rhodium, palladium, rhenium, osmium, iridium, and platinum. The transition metals used to provide intermetallic compounds are typically titanium, vanadium, chromium, manganese, iron, cobalt, nickel, zirconium, columbium, molybdenum, hafnium, tantalum, tungsten, tin, and lead. The intermetallic compositions are typically stoichiometric compounds having a unique formula, x-ray diffraction pattern, and physical properties.
DETAILED DESCRIPTION OF THE INVENTION
It has now been found that a particularly desirable anode may be provided by an electroconductive substrate having a layer of an electroconductive intermetallic compound of a platinum group metal and a transition metal thereon. The platinum group metals used in providing the intermetallic composition include ruthenium, rhodium, palladium, rhenium, osmium, iridium, and platinum. The transition metals used to provide the intermetallic composition are titanium, vanadium, chromium, manganese, iron, cobalt, nickel, zirconium, columbium, molybdenum, hafnium, tantalum, tungsten, tin, and lead. Particularly preferred transition metals are the refractory metals, i.e., titanium, vanadium, chromium, zirconium, columbium, molybdenum, tantalum, tungsten, hafnium, and manganese.
The platinum group metal and transition metal of the intermetallic composition are in a stoichiometric relationship. That is, the platinum group metal and the transition metal are in a small whole number molar ratio to each other in the intermetallic compositions. Most generally they are in a ratio of from about 1 to about 7 atoms of the precious metal to about 1 to about 7 atoms of the transition metal and generally from about 1 to about 3, 4, or 5 atoms of the precious metal to about 1 to about 3, 4, or 5 atoms of the transition metal. Additionally, stoichiometric relationships may be determined by x-ray diffraction patterns occurring within a narrow vicinity of a small whole number mole ratio. According to the metallurgical literature, this is indicative of a unique crystal structure in the vicinity of the small whole number mole ratio.
Furthermore, the intermetallic compositions useful in providing the electrode material of this invention are characterized by a local maximum or minimum of physical and physicochemical properties with respect to composition. For example, the melting point may be a maximum such as a congruent melting composition or a minimum such as a eutectic composition. Additionally, within the narrow range of stoichiometric compositions herein contemplated, the electroconductivity may be a relative maximum with respect to composition. The hardness and impact strength may also be relative maximums with respect to composition. Furthermore, upon microscopic examination, for example, by polishing a surface of a casting of a material and etching the polished surface in a suitable acid or other reagent, the substantial presence of only a single metallographic phase may be detected. Additionally, a congruent solid phase transformation, such as a peritectoid, may be the composition within the narrow stoichiometric compositions herein contemplated.
The small whole number molar ratio stoichiometry, the unique x-ray diffraction patterns, the local extrema of physical and physicochemical properties, and the presence of only a single metallographic phase, either singly or in combination, give rise to the characterization of the intermetallic compositions as being intermetallic compounds. Whenever the term "intermetallic compound" is used herein, it will be understood to refer to an intermetallic composition characterized by at least some of the following properties: a small whole number molar ratio of the precious metal and transition metal, a unique x-ray diffraction pattern within a narrow range of the small whole number molar ratio, local extrema of physical and physicochemical properties within a narrow range of the small whole number molar ratio, and the presence of only a single matallographic phase.
The intermetallic compositions useful in providing the anodic materials herein contemplated have the formula:
M'.sub.A M".sub.B
where M' may be ruthenium, rhodium, palladium, osmium, iridium, platinum, and rhenium; M" may be titanium, vanadium, zirconium, niobium, columbium, pg,5 molybdenum, tantalum, tungsten, iron, chromium, nickel, manganese, tin, and lead; A is from 1 to about 7; and B is from about 1 to about 7.
Typical ruthenium compounds useful in providing the electrode material of this invention include RuTi, RuV, Ru.sub.2 Zr, NbRu, Mo.sub.5 Ru.sub.3, RuTa, W.sub.3 Ru.sub.2, RuCr.sub.2, and Sn.sub.2 Ru. Typical rhodium compositions useful in providing the electrode material of this invention include Rh.sub.3 Ti, Rh.sub.3 V, Rh.sub.3 Zr, Rh.sub.3 Nb, Rh.sub.3 Ta, RhCr.sub.3, Rh.sub.3 Sn, and Rh.sub.3 Pb.sub.2. Typical palladium compositions useful in providing the electrode material of this invention include TiPd.sub.3, Pd.sub.3 V, Pd.sub.3 Zr, PdTa, FePd.sub.3, Pd.sub.3 Mn.sub.2, PdSn.sub.3, and Pd.sub.3 Pb. Typical osmium compositions useful in providing the electrode material of this invention include OsTi, OsV, OsZr, Nb.sub.3 Os.sub.2, Mo.sub.19.5 Os.sub.10.5, Ta.sub.3 Os, WOs.sub.2, and Cr.sub.2 Os. Typical iridium compositions useful in providing the electrode material of this invention include TiIr.sub.3, VIr.sub.3, ZrIr.sub.2, Ir.sub.3 Nb, Mo.sub.3 Ir, TaIr.sub.3, HfIrNi, Cr.sub.3 Ir, Mn.sub.3 Ir, and IrSn.sub.2. Typical platinum compositions useful in providing the electrode composition of this invention include Pt.sub.3 Ti, Pt.sub.3 V, Pt.sub.3 Zr, Pt.sub.3 Nb, Pt.sub.3 Mo, Pt.sub.4 Ta, Pt.sub.3 Fe, PtCr.sub.2, Pt.sub.2 Sn.sub.3, PbPt.sub.5, PbPt.sub.6, and PbPt.sub.7. Typical rhenium compositions useful in providing the electrode material of this invention include Ti.sub..9 Re.sub..1 Si.sub.2, Re.sub.3 V, Re.sub.2 Zr, NbRe, MoRe, TaRe, WRe, Re.sub.3 Fe.sub.2, Re.sub.6 Co.sub.5.7 Si.sub.1.3, CrRe, and Mn.sub.3 Re.sub.2. The particulaly preferred compositions are the compositions containing titanium, tantalum, and molybdenum.
The particularly preferred rhenium compositions include Ti.sub..9 Re.sub..1 Si.sub.2, MoRe, and TaRe. The particularly preferred rhodium compositions include Rh.sub.3 Ti and Rh.sub.3 Ta. The particularly preferred palladium compositions include TiPd.sub.3 and PdTa.
The particularly preferred osmium compositions include OsTi, Mo.sub.19.5 Os.sub.10.5, and Ta.sub.3 Os.
The particularly preferred iridium compositions include TiIr.sub.3, Mo.sub.3 Ir, and TaIr.sub.3.
The particularly preferred platinum compositions include Pt.sub.3 Ti, Pt.sub.3 Mo, and Pt.sub.4 Ta. Especially preferred compositions are the molybdenum containing compositions Mo.sub.5 Ru.sub.3, Mo.sub.14.5 Os.sub.10.5, Mo.sub.3 Ir, Pt.sub.3 Mo, and MoRe.
Typical titanium compositions useful in providing the electrode material of this invention include RuTi, Rh.sub.3 Ti, TiPd.sub.3, OsTi, TiIr.sub.3, Pt.sub.3 Ti, and Ti.sub..9 Re.sub..1 Si.sub.2. Typical vanadium compositions useful in providing the electrode material of this invention include RuV, Rh.sub.3 V, Pd.sub.3 V, OsV, VIr.sub.3, Pt.sub.3 V, and Re.sub.3 V. Typical zirconium compositions useful in providing the electrode material of this invention include Ru.sub.2 Zr, Rh.sub.3 Zr, Pd.sub.3 Zr, OsZr, ZrIr.sub.2, Pt.sub.3 Zr, and Re.sub.2 Zr. Typical columbium (niobium) compositions useful in providing the electrode material of this invention include NbRu, Rh.sub.3 Nb, Nb.sub.3 Os.sub.2, Ir.sub.3 Nb, Pt.sub.3 Nb, and NbRe. Typical molybdenum compositions useful in providing the electrode material of this invention include Mo.sub.5 Ru.sub.3, Mo.sub.14.5 Os.sub.10.5, Mo.sub.3 Ir, Pt.sub.3 Mo, and MoRe. Typical tantalum compositions useful in providing the electrode materials of this invention include RuTa, Rh.sub.3 Ta, PdTa, Ta.sub.3 Os, TaIr.sub.3, Pt.sub.4 Ta, and TaRe. Typical tungsten compositions useful in providing the electrode materials of this invention include W.sub.3 Ru.sub.2, WOs.sub.2, and WRe. Typical iron compositions useful in providing the electrode materials of this invention include FePd.sub.3, Pt.sub.3 Fe, and Re.sub.3 Fe.sub.2. A nickel composition useful in providing the electrode material of this invention is HfIrNi. Typical chromium compositions useful in providing the electrode materials of this invention include RuCr.sub.2, RhCr.sub.3, Cr.sub.2 Os, Cr.sub.3 Ir, PtCr.sub.2, and CrRe. Typical manganese compositions useful in providing the electrode materials of this invention include Pd.sub.3 Mn.sub.2, Mn.sub.3 Ir, and Mn.sub.3 Re.sub.2. Typical tin compositions useful in providing the electrode materials of this invention include Sn.sub.2 Ru, Rh.sub.3 Sn, PdSn.sub.3, IrSn.sub.2, and Pt.sub.2 Sn.sub.3. Typical lead compositions useful in providing the electrode materials of this invention include RhPb.sub.2, Pd.sub.3 Pb, PbPt.sub.5, PbPt.sub.6, and PbPt.sub.7.
One class of intermetallic compositions useful in providing the electrode materials of this invention are those having a tetragonal crystal habit with a P.sub.4.sbsb.2 space group. This class of materials includes the intermetallic compositions of molybdenum and ruthenium, Mo.sub.5 Ru.sub.3, of platinum and columbium, Pt.sub.3 Nb, of tantalum and osmium, Ta.sub.3 Os, of chromium and osmium, Cr.sub.2 Os, and of vanadium, Re.sub.3 V, niobium, NbRe, tantalum, TaRe, tungsten, WRe, chromium, CrRe, and manganese, MnRe, compounds of rhenium, chromium and iridium, Cr.sub.3 Ir, and manganese and iridium, Mn.sub.3 Ir. Typical x-ray diffraction patterns reported in the A.S.T.M. X-Ray Powder Diffraction Files are shown in Tables I through III.
Table I______________________________________X-Ray Diffraction Pattern of OsTaInterplanar Spacing (d) IntensityAngstroms (I/I.sub.1)______________________________________2.586 202.400 752.335 402.235 352.179 1002.129 702.082 201.393 301.380 401.354 401.342 301.318 30______________________________________
Table II______________________________________X-Ray Diffraction Pattern of Cr.sub.2 OsInterplanar Spacing (d) IntensityAngstroms (I/I.sub.1)______________________________________4.13 52.472 102.358 202.217 702.151 202.092 202.037 452.002 1001.954 551.899 10______________________________________
Table III______________________________________X-Ray Diffraction Pattern of Mo.sub.5 Ru.sub.3Interplanar Spacing (d) IntensityAngstroms (I/I.sub.1)______________________________________2.58 202.48 202.32 602.26 202.21 302.10 1002.05 302.00 201.91 301.61 301.44 301.37 601.34 601.31 601.27 601.25 601.23 601.17 501.16 501.15 501.13 601.09 601.07 501.05 601.02 601.00 300.06 20.95 30.93 50.92 50.91 50.90 70.88 70.87 50.865 50.860 50.856 50.851 30.846 70______________________________________
Table I is the data for Ta.sub.3 Os reported by Nevitt and Downey, Trans. AIME, 209, 1057 (1957) and reproduced in A.S.T.M. X-Ray Powder Diffraction File, 10-293. Table II is from the data of Waterstrat and Kasper, J. Metals, 9, 872 (1957), reproduced in A.S.T.M. X-Ray Powder Diffraction File, 9-319, for Cr.sub.2 Os. Table III is from the data of Raub, Z. Metallkunde, 45, 23 (1954), and Greenfield and Beck, J. Met. 8, 265 (1956), both reported in A.S.T.M. X-Ray Powder Diffraction File, 7-129, for Mo.sub.5 Ru.sub.3.
Another desirable category of materials are those having a cubic crystal structure with a P.sub.m 3.sub.n space group. Included within this category are the compositions of titanium, i.e., rhodium titanium, Rh.sub.3 Ti, platinum titanium, Pt.sub.3 Ti, and iridium titanium, Ir.sub.3 Ti, the compositions of vanadium such as rhodium vanadium, Rh.sub.3 V, ruthenium vanadium, RuV, iridium vanadium, Ir.sub.3 V, and osmium vanadium, OsV, the zirconium compounds having a cubic structure rhodium zirconium, Rh.sub.3 Zr, ruthenium zirconium, RuZr, and osmium zirconium, OsZr. Also included within this category are the compositions of iridium tantalum, Ir.sub.3 Ta, rhodium tantalum, Rh.sub.3 Ta, and platinum chromium, PtCr.sub.2. Typical x-ray diffraction patterns are shown in Tables IV through XIX.
Table IV______________________________________X-Ray Diffraction Pattern of RuTiInterplanar Spacing (d) IntensityAngstroms (I/I.sub.1)______________________________________3.051 302.161 1001.769 101.530 401.370 101.251 701.083 301.022 100.970 60.925 10.886 30.851 10.8203 100______________________________________
Table V______________________________________X-Ray Diffraction Pattern of Rh.sub.3 TiInterplanar Spacing (d) IntensityAngstroms (I/I.sub.1)______________________________________3.80 52.680 52.188 1001.906 901.699 51.555 101.350 501.273 101.206 51.151 901.100 601.059 51.021 50.955 60.927 5.901 5.8768 70.8545 50.8335 5.8146 5.7801 90______________________________________
Table VI______________________________________X-Ray Diffraction Pattern of Pt.sub.3 TiInterplanar Spacing (d) IntensityAngstroms (I/I.sub.1)______________________________________3.87 102.743 202.244 1001.946 701.742 151.591 151.379 901.300 151.235 101.177 801.127 701.084 51.043 150.976 40.946 10.919 5.896 100.8732 70.8519 5.8327 15.7973 100.7810 5______________________________________
Table VII______________________________________ X-Ray Diffraction Pattern of Ir.sub.3 Ti______________________________________Interplanar Spacing (d) IntensityAngstroms (I/I.sub.1)______________________________________3.80 152.695 52.208 1001.915 801.713 101.567 51.355 801.279 101.212 51.157 1001.107 401.060 51.027 100.961 30.932 10.906 5.8817 100.8595 100.8387 15.8195 5.7848 100______________________________________
Table VIII______________________________________ X-Ray Diffraction Pattern of RuZr______________________________________Interplanar Spacing (d) IntensityAngstroms (I/I.sub.1)______________________________________2.288 1001.619 401.324 701.148 401.026 600.938 20.869 100.813 30______________________________________
Table IX______________________________________ X-Ray Diffraction Pattern of Rh.sub.3 Zr______________________________________Interplanar Spacing (d) IntensityAngstroms (I/I.sub.1)______________________________________2.245 1001.947 701.383 801.181 901.130 400.980 30.900 100.977 100.8013 100______________________________________
Table X______________________________________ X-Ray Diffraction Pattern of OsZr______________________________________Interplanar Spacing (d) IntensityAngstroms (I/I.sub.1)______________________________________3.229 202.292 1001.875 101.626 401.455 201.329 801.152 401.087 101.031 700.982 10.941 30.904 10.872 100.8155 40.7912 50______________________________________
Table XI______________________________________X-Ray Diffraction Pattern of Rh.sub.3 VInterplanar Spacing (d) IntensityAngstroms (I/I.sub.1)______________________________________3.75 52.672 52.175 1001.887 801.689 101.541 51.338 901.260 101.192 51.143 1001.091 501.047 51.009 50.947 40.919 5.893 10.8703 100.8483 100.8278 5.8089 5.7746 100______________________________________
Table XII______________________________________X-Ray Diffraction Pattern of RuVInterplanar Spacing (d) IntensityAngstroms (I/I.sub.1)______________________________________2.985 302.115 1001.729 201.498 601.341 301.224 801.060 501.000 300.949 80.905 10.866 40.832 20.8022 >100______________________________________
Table XIII______________________________________X-Ray Diffraction Pattern of OsVInterplanar Spacing (d) IntensityAngstroms (I/I.sub.1)______________________________________2.992 502.121 1001.733 301.502 501.345 401.228 801.063 401.002 300.952 60.907 30.868 40.834 30.8044 100______________________________________
Table XIV______________________________________X-Ray Diffraction Pattern of Ir.sub.3 VInterplanar Spacing (d) IntensityAngstroms (I/I.sub.1)______________________________________3.74 152.670 152.182 1001.896 901.694 151.550 151.342 901.268 151.202 101.147 1001.108 401.055 51.015 100.951 30.923 5.897 10.873 100.8514 100.8315 10.8123 10.7780 100______________________________________
Table XV______________________________________X-Ray Diffraction Pattern of Ir.sub.3 NbInterplanar Spacing (d) IntensityAngstroms (I/I.sub.1)______________________________________3.83 102.743 102.238 1001.940 801.736 101.584 101.371 901.294 101.228 51.170 1001.121 501.077 51.038 100.972 40.944 5.917 5.893 80.8699 90.8493 5.8298 15.7946 100______________________________________
Table XVI______________________________________X-Ray Diffraction Pattern of Ir.sub.3 TaInterplanar Spacing (d) IntensityAngstroms (I/I.sub.1)______________________________________2.24 1001.938 901.371 901.170 1001.121 400.971 30.892 100.8690 100.7935 100______________________________________
Table XVII______________________________________X-Ray Diffraction Pattern of Cr.sub.3 RhInterplanar Spacing (d) IntensityAngstroms (I/I.sub.1)______________________________________3.30 252.337 552.090 801.909 1001.652 41.478 41.349 41.296 181.249 401.168 141.102 21.045 101.020 100.996 10.954 2.917 2.868 12.853 14.826 10.802 2______________________________________
Table XVIII______________________________________X-Ray Diffraction Pattern of PtCr.sub.2Interplanar Spacing (d) IntensityAngstroms (I/I.sub.1)______________________________________2.67 702.185 1001.896 801.698 401.549 101.343 1001.260 40______________________________________
Table XIX______________________________________X-Ray Diffraction Pattern of Cr.sub.3 IrInterplanar Spacing (d) IntensityAngstroms (I/I.sub.1)______________________________________3.309 802.3404 552.0937 551.9106 1001.6554 101.4805 121.2978 61.2506 451.1700 101.1031 81.0466 121.0216 40.9979 12.9556 4.9180 10.8693 4.8546 14.8275 8.8028 6______________________________________
Table IV shows the data for RuTi reported in A.S.T.M., X-Ray Powder Diffraction File, Card 18-1144. Table V shows the data of Downey, Met. Div. Argonne Nat. Lab., Argonne, Ill. (1964) and of Dwight and Beck, Trans. AIME, 215, 976 (1959), reproduced in A.S.T.M., X-Ray Powder Diffraction File, Card 17-50, for Rh.sub.3 Ti. Table VI shows the data for Pt.sub.3 Ti and Table VII shows the data for Ir.sub.3 Ti, reported by Downey, Met. Div., Argonne Nat. Lab., Argonne, Ill., (1964) and by Dwight and Beck, Trans. AIME, 215, 976 (1959), reproduced in A.S.T.M., X-Ray Powder Diffraction File, Cards 17-64 (Pt.sub.3 Ti) and 17-37 (Ir.sub.3 Ti). Table VIII shows the data reported in A.S.T.M., X-Ray Powder Diffraction File, Card 18-1147, for RuZr. Table IX shows the data of Downey, op cit, and of Dwight and Beck, op cit, for Rh.sub.3 Zr, reproduced in A.S.T.M., X-Ray Powder Diffraction File, Card 17-49. Table X shows the data for OsZr reported in A.S.T.M., X-Ray Powder Diffraction File, Card 18-949. Table XI shows the data of Downey, op cit, and of Dwight and Beck, op cit, for Rh.sub.3 V, reproduced in A.S.T.M., X-Ray Powder Diffraction File, Card 17-63. Table XII shows the data for RuV reported in A.S.T.M., X-Ray Powder Diffraction File, Card 19-1111. Table XIII shows the data for OsV reported in A.S.T.M., X-Ray Powder Diffraction File, Card 18-1947. Table XIV shows the data of Downey, op cit, and Dwight and Beck, op cit, for Ir.sub.3 V, reported in A.S.T.M., X-Ray Powder Diffraction Files, Card 17-38. Table XV shows the data of Downey, op cit, and of Dwight and Beck, op cit, for Ir.sub.3 Nb, reported in A.S.T.M., X-Ray Powder Diffraction Files, Card 17-35. Table XVI shows the data of Downey, op cit, and of Dwight and Beck, op cit, for Ir.sub.3 Ta, reported in A.S.T.M., X-Ray Powder Diffraction Files, Card 17-36. Table XVII shows the data of Monograph 25, Section 6, 15, National Bureau of Standards (1968), for Cr.sub.3 Rh, reported in A.S.T.M., X-Ray Powder Diffraction Files, Card 20-314. Table XVIII shows the data of Greenfield and Beck, J. Metals (Feb.) 265 (1956), for PtCr.sub.2, reproduced in A.S.T.M., X-Ray Powder Diffraction Files, Card 8-340. Table XIX shows the data of Monograph 25, Sec. 6, National Bureau of Standards (1968), for Cr.sub.3 Ir, reported in A.S.T.M., X-Ray Powder Diffraction File, Card 18-385.
Three other molybdenum compounds in addition to Mo.sub.5 Ru.sub.3 and Mo.sub.3 Ir that are useful in providing the electrode material of this invention are Pt.sub.3 Mo, Mo.sub.19.5 Os.sub.10.5, and MoRe. Table XX shows the data of Rooksby and Lewis, J. Less Common Metals 6, 451-60 (1964), reproduced in A.S.T.M., X-Ray Powder Diffraction File, Card 17-719, for Pt.sub.3 Mo. Table XXI shows the data of Spooner and Wilson, Acta Cryst., 17, 12, 1533-38, reproduced in A.S.T.M., X-Ray Powder Diffraction File, Card 18-843, for Mo.sub.19.5 Os.sub.10.5. Table XXII shows the data of Tylkina et al, Proc. Acad. Sci., U.S.S.R., 131, 247, reproduced in A.S.T.M., X-Ray Powder Diffraction File, Card 15-116, for MoRe.
Table XX______________________________________X-Ray Diffraction Pattern of Pt.sub.3 MoInterplanar Spacing (d) IntensityAngstroms (I/I.sub.1)______________________________________3.85 52.580 152.257 1001.968 151.949 351.575 101.525 101.385 501.378 501.308 51.285 151.185 251.176 601.129 351.088 51.062 50.990 5.984 10.974 20.898 70.895 30.878 20.873 40.871 40.828 10.821 10.801 15.797 30.777 10______________________________________
Table XXI______________________________________X-Ray Diffraction Pattern of Mo.sub.19.5 Os.sub.10.5Interplanar Spacing (d) IntensityAngstroms (I/I.sub.1)______________________________________4.40 204.30 44.00 63.40 23.24 22.677 42.470 252.327 852.266 352.197 252.145 652.109 1002.060 601.999 101.920 161.812 81.792 81.762 101.723 21.696 41.679 41.633 41.499 21.447 41.371 161.360 81.345 301.332 20______________________________________
Table XXII______________________________________X-Ray Diffraction Pattern of MoReInterplanar Spacing (d) IntensityAngstroms (I/I.sub.1)______________________________________2.308 1002.247 752.202 252.142 502.097 1002.044 75______________________________________
The preferred intermetallic compositions useful in providing anodes and carrying out electrochemical processes in electrolytic cells are high melting point compositions having a melting point range of from about 700.degree. C to about 2600.degree. C, most frequently in the range of from about 1000.degree. C to about 2000.degree. C for compositions of titanium, in the range of about 700.degree. C to about 1600.degree. C for compositions of vanadium, in the range of about 1200.degree. C to about 2000.degree. C for compositions of chromium, in the range of about 1000.degree. C to about 2200.degree. C for compositions of zirconium, in the range of about 1100.degree. C to about 2000.degree. C for compositions of columbium, in the range of about 1300.degree. C to about 2600.degree. C for compositions of molybdenum, in the range of about 1800.degree. C to about 2600.degree. C for compositions of tantalum, in the range of about 1800.degree. C to about 2600.degree. C for compositions of tungsten, and in the range of about 1100.degree. C to about 1600.degree. C for compositions of manganese.
Additionally, the preferred intermetallic compositions of platinum group metals and transition metals have a bulk electrical conductivity of from about 10.sup.-.sup.1 (ohm-centimeters).sup.-.sup.1 to about 10.sup.- 6 (ohm-centimeters).sup.-.sup.1.
A preferred category of the intermetallic compositions of platinum group metals and transition metals are those characterized by a low, steady state chlorine overvoltage, i.e., by a chlorine overvoltage of less than 0.30 volts (300 millivolts) after the transients have approached zero. Typically, steady state is attained at a time of from about several minutes after commencing the test for 3 or 4 days after commencing tests. The chlorine overvoltage is determined as follows:
A two-compartment cell constructed of polytetrafluoroethylene with a diaphragm composed of asbestos paper is used in the measurement of chlorine overpotentials. A stream of water-saturated Cl.sub.2 gas is dispersed into a vessel containing saturated NaCl, and the resulting Cl.sub.2 -saturated brine is continuously pumped into the anode chamber of the cell. In normal operation, the temperature of the electrolyte ranges from 30.degree. to 35.degree. C, most commonly 32.degree. C, at a pH of 4.0. A platinized titanium cathode is used.
In operation, an anode is mounted to a titanium holder by means of titanium bar clamps. Two electrical leads are attached to the anode; one of these carries the applied current between anode and cathode at the voltage required to cause continuous generation of chlorine. The second is connected to one input of a high impedance voltmeter. A Luggin tip made of glass is brought up to the anode surface. This communicates via a salt bridge filled with anolyte with a saturated calomel half cell. Usually employed is a Beckman miniature fiber junction calomel such as catalog No. 39270, but any equivalent one would be satisfactory. The lead from the calomel cell is attached to the second input of the voltmeter and the potential read.
Calculation of the overvoltage, .eta., is as follows:
The International Union of Pure and Applied Chemistry sign convention is used, and the Nernst equation taken in the following form: ##EQU1##
Concentrations are used for the terms in brackets instead of the more correct activities.
E.sub.0 = the standard state reversible potential = +1.35 volts
n = number of electrons equivalent .sup.-.sup.1 = 1
R, gas constant, = 8.314 joule deg.sup.-.sup.1 mole.sup.- .sup.1
F, the Faraday = 96,500 couloumbs equivalent.sup.-.sup.1
Cl.sub.2 concentration = 1 atm
Cl.sup.- concentration = 5.4 equivalent liter.sup.-.sup.1 (equivalent to 305 grams NaCl per liter)
T = 305.degree. k
for the reaction
Cl.sup.-.fwdarw.1/2 Cl.sub.2 + e.sup.-,
E = 1.35 + 0.060 log 1/5.4 = 1.30
This is the reversible potential for the system at the operating conditions. The overvoltage on the normal hydrogen scale is, therefore,
.eta. = V - [E - 0.24]
where V is the measured voltage, E is the reversible potential (1.30 volts) and 0.24 volt is the potential of the saturated calomel half cell.
According to a preferred exemplification of this invention, the electrode has the active electrocatalytic surface made up of the intermetallic composition of the platinum group metal and the transition metal. The surface may consist essentially of the preferred intermetallic compounds or the surface may comprise the intermetallic compound admixed with refractory materials as TiO.sub.2, ZrO.sub.2, and the like, in which case the surface may contain from about five to about ninety-five weight percent of the intermetallic composition and balance refractory material. Alternatively, the intermetallic composition of the platinum group metal and the transition metal may be provided on the surface of the substrate and a further catalytic material such as a surface catalyst, or an electrocatalyst, or both, may be on the exterior of the electrode. In such a case, the intermetallic compositions herein contemplated serve as an electroconductive sheet or shield or sheath for the substrate, allowing the use of copper, aluminum, iron, or steel substrates for anodes.
In such a case, the catalytic surface atop the intermetallic compositions herein contemplated may be provided by surface catalysts such as spinels, perovskites, bronze oxides, and the like. Alternatively, the material may be provided by various oxides and oxygen-containing compounds of the platinum group metals such as calcium ruthenate, calcium rhodate, strontium ruthenate, strontium rhodate, pyrochlores, delafossites, and mixtures thereof.
The substrate is characterized by an electrical conductivity sufficient to allow its economical use in an electrolytic cell, generally in excess of 100 (ohm-centimeters).sup.-.sup.1 and preferably in excess of 1000 (ohm-centimeters).sup.-.sup.1. The substrate may be graphite. Alternatively, the substrate may be a valve metal. By "valve metals" are meant those metals which form an electrically insulating, corrosion resistant film upon exposure to acidic media under anodic conditions. The valve metals include titanium, zirconium, hafnium, vanadium, columbium, tantalum, tungsten, alloys thereof and alloys predominant in any of the above-mentioned valve metals. Where the substrate is a valve metal, the platinum group metal may be deposited on the surface thereof and reacted with the substrate to form the compositions herein contemplated at the interface between the platinum group metal and the valve metal. However, care must be taken to form the desired compound and not a high overvoltage compound or a high resistance alloy or compound.
Alternatively, the substrate may be silicon in which case the intermetallic compositions herein contemplated provide an electroconductive, electrocatalytical coating thereon.
Where an intermetallic composition, e.g., a higher overvoltage intermetallic composition, is used as an intermediate layer or coating as herein contemplated, the high overvoltage intermetallic composition may provide an electroconductive film, layer, coating, lamination, or sheet upon a substrate fabricated of material that is susceptible to corrosion by acidic media under anodic conditions. For example, a substrate of iron, steel, copper, or aluminum, may be used with an intermediate layer of an electroconductive corrosion resistant composition such as described herein. In this case, the copper or aluminum or iron or steel core may have a thin coating, for example, as thin as one-one thousandth of an inch (2.54 .times. 10.sup.-.sup.2 mm) or less, of the intermetallic compositions herein contemplated. Such a thin coating must be of sufficient thickness to protect the core from the corrosive environment but may still be thin enough to be a low cost method of using inexpensive metals in fabricating chlor-alkali anodes.
Thereafter, an exterior coating or coatings of an electrocatalytic material or materials may be provided atop the intermediate of an intermetallic composition as herein contemplated. Such exterior coating or coatings may be characterized by some porosity, i.e., by a high degree of surface per unit mass whereby both electrocatalytic and surface-catalytic effects may be provided. For example, one particularly desirable anode may be provided by a foraminous or perforate or expanded mesh aluminum substrate having a coating of about one-one thousandth to about one hundredth of an inch thick (2.54 .times. 10.sup.-.sup.2 mm to about 2.54 .times. 10.sup.-.sup.1 mm) thereon of an intermetallic composition of the type described hereinabove, such as a ruthenium molybdenum compound (Mo.sub.5 Ru.sub.3) or a platinum titanium compound (Pt.sub.3 Ti) or any of the materials as hereinabove described. Atop the intermediate protective layer of the intermetallic composition of the platinum group metal and the transition metal is a further coating, for example, a ruthenium dioxide-titanium dioxide composition, or a delafossite, or a pyrochlore, or an intermetallic composition of a platinum group metal and a transition metal, characterized by a low overvoltage.
According to another exemplification of this invention, a suitable anode for chlor-alkali electrolysis may be provided by a foraminous or perforate or mesh or rod-like titanium substrate having as its electrolytic surface a thin surface, e.g., of from about one-one thousandth of an inch to about one-one hundredth of an inch (2.54 .times. 10.sup.-.sup.2 mm to about 2.54 .times. 10.sup.-.sup.1 mm) thick of an intermetallic composition of the type herein contemplated such as Pt.sub.3 Mo or Ru.sub.3 Mo.sub.5 or PdTa or Pt.sub.3 Ti or Mo.sub.3 Ir or MoRe.
The anodes prepared according to this invention may be used in electrolytic cells, for example, diaphragm cells or mercury cells for the production of chlorine or chlorate cells. A typical diaphragm cell for the production of chlorine would include an anode of the type herein contemplated, having a layer or surface of an intermetallic composition of a platinum group metal and a transition metal on a suitable electroconductive substrate as the anode, a suitable cathode, for example, an iron mesh or steel mesh cathode, and a diaphragm therebetween, for example, an asbestos diaphragm deposited on the cathode or an asbestos paper diaphragm between the anode and the cathode, or even a permionic membrane interposed between the anode and the cathode.
In such a diaphragm cell, brine is fed to the anolyte chamber of the cell and an electrical current is caused to pass from the anode of the cell through the electrolyte to the cathode of the cell, evolving chlorine on the anodes and hydrogen on the cathodes. Within a diaphragm cell, chlorine is recovered from the anolyte chamber and hydrogen gas is recovered from the catholyte chamber. Where the diaphragm is an electrolyte permeable diaphragm, a cell liquor of sodium chloride and sodium hydroxide is recovered from the catholyte chamber. In an electrolytic cell where a permionic membrane is interposed between the anolyte chamber and catholyte chamber, a catholyte liquor consisting essentially of aqueous sodium hydroxide is recovered. While the process of diaphragm cell has been described with reference to sodium chloride brines, it is to be understood that potassium chloride brines may also be electrolyzed with the anodes herein described.
Additionally, the anodes of the type herein contemplated may be used in mercury cells. Mercury cells are characterized by a downward sloping electroconductive surface having a thin film of mercury flowing thereon. The electrolyte, e.g., brine, flows atop the mercury surface with the anodes extending downwardly into the electrolyte and spaced from the mercury surface. Within a mercury cell, chlorine is evolved on the anodes and sodium or potassium metal is amalgamated with the mercury. The mercury is recovered from the cell and transferred to a de-nuder where the mercury sodium or mercury potassium amalgam is contacted with water whereby to evolve an alkali metal hydroxide such as sodium hydroxide or potassium hydroxide.

The details of the invention herein contemplated may be understood by reference to the following illustrative Examples.
EXAMPLE I
A triplatinum molybdenum (Pt.sub.3 Mo) anode was prepared and tested.
The intermetallic composition of platinum and molybdenum was prepared by grinding 1.3250 grams of Engelhard 24 gauge platinum sheet and 0.2175 grams of 0.00015 inch (3.81 .times. 10.sup.-.sup.6 m) molybdenum sheet in a Mullite (R) mortar and pestle. The ground platinum and molybdenum were pressed into pellets in a one-half inch diameter (1.27 cm. diameter) pan pellet press. The pellets were then inserted in suitable mold for fusion. The fusion mold was prepared from an 8 inch by 8 inch by 2 inch (20.4 cm. by 20.4 cm. by 5.08 cm.) graphite block by drilling four cups 1 inch in diameter by 5/8 inch deep (2.54 cm. diameter by 1.59 cm. deep) in the graphite block. The pellets were then placed in the cup and melted using a tungsten electrode arc welder under an Argon atmosphere. Fusion occurred at a current of 75 amperes within 2 to 3 seconds and the current was continued from 3 to 5 seconds after fusion.
The resulting metal alloy beads were tested for electrical conductivity and overvoltage in aqueous sodium chloride solution. This was accomplished by fastening two copper wires to the back of the Pt.sub.3 Mo bead using an epoxy resin loaded with silver. This assembly was cast in Quick-Set (R) plastic. The face of the plastic bead was ground to expose the alloy bead. The alloy bead was polished and the copper wires were sheathed in Teflon (R) tubing. The side of the assembly away from the Pt.sub.3 Mo bead, where the copper tubing entered the plastic, was set using Silastic (R) cement.
The resulting assembly was then tested as an anode. Chlorine was seen to be evolved and no corrosion was observed.
EXAMPLE II
A ruthenium molybdenum alloy having the formula Ru.sub.3 Mo.sub.5 was prepared and tested as an anode.
The Ru.sub.3 Mo.sub.5 alloy was prepared from 0.9096 grams of 60 mesh Alpha Inorganics ruthenium powder and 1.4379 grams of Cerac 2-4 molybdenum powder.
The ruthenium powder and molybdenum powder were ground together in a Mullite mortar and pestle. The resulting ground powder was then pressed into pellets in a one-half inch diameter (1.27 cm. diameter) pan pellet press. The pellets were then placed in a graphite block as described in Example I hereinabove. The pellets were then arc melted using a tungsten electrode arc welder under an Argon atmosphere. The welding current was 150 amperes for 2 to 3 seconds until fusion was obtained and then an additional 3 to 5 seconds.
The resulting Ru.sub.3 Mo.sub.5 bead was tested for electrical conductivity and overvoltage in an aqueous sodium chloride solution under anodic conditions. Two copper wires were fastened to the back of the bead using silver loaded epoxy resin. The assembly of the bead and copper wires was then cast in Quick-Set (R) plastic. The face of the assembly was ground down to expose the Ru.sub.3 Mo.sub.5 alloy bead and the alloy bead was polished. The copper wires were sheathed in Teflon (R) tubing and Silastic (R) cement as described hereinabove.
The alloy was tested as an electrode in aqueous sodium chloride solution and chlorine gas was seen to be evolved. No corrosion was observed and the overvoltage was found to be 0.11 volts at a current density of 200 amperes per square foot.
EXAMPLE III
A palladium tantalum alloy, PdTa, was prepared and tested as an anode for a chlorine evolution.
The PdTa alloy was prepared by mixing 0.8512 gram of Engelhard 200 mesh palladium powder and 1.4476 grams of Alpha Inorganics 325 mesh tantalum powder in a mortar and pestle. The palladium and tantalum powders were ground together in a Molite mortar pestle. Thereafter, the mixed palladium and tantalum powders were pressed into pellets in a one-half inch diameter (1.27 cm. diameter) pan pellet press. The resulting pellet was placed in a cup in a graphite block as described in Example I hereinabove. The palladium and tantalum powders in the cup within the graphite block were then arc melted using a tungsten electrode arc under an Argon atmosphere. The arc melting current was 75 amperes and fusion was obtained after 2 to 3 seconds. Power was continued for an additional 3 to 5 seconds.
The resulting PdTa beads were tested for electrical conductivity in aqueous sodium chloride under anodic conditions. In preparing the test assembly, two copper wires were fastened to the back of the PdTa bead using a silver lead of epoxy resin. This assembly was then cast in Quick-Set (R) plastic and the face ground down to expose the alloy bead as described in Example I hereinabove. The alloy bead was thereafter polished and the copper wires sheathed with Teflon (R) tubing. The face where the copper wires entered the plastic was set using Silastic (R) cement as described hereinabove.
This palladium tantalum anode assembly was then tested as an anode in aqueous sodium chloride. Chlorine was seen to be evolved and no corrosion was observed.
EXAMPLE IV
A platinum titanium, Pt.sub.3 Ti, anode was prepared and tested.
The Pt.sub.3 Ti was prepared from 1.2718 grams of Engelhard 24 gauge platinum sheet and 0.1150 grams of 1/16 inch (1.59 mm.) titanium sheet. The platinum sheet and titanium sheet were ground together in a Mullite (R) mortar and pestle. The platinum and titanium were then removed from the mortar and pestle and pressed into pellets in 1/2 inch diameter (1.27 cm.) pan pellet press.
The pellets were then inserted in cups in a graphite block as described in Example I hereinabove and arc melted using a tungsten electrode arc welder under an Argon atmosphere. The arc melting current was 75 amperes and fusion was obtained in 2 to 3 seconds with an additional 3 to 5 seconds of current.
The resulting Pt.sub.3 Ti beads were tested for electrical conductivity in aqueous sodium chloride under anodic conditions. An anode assembly was prepared as described above by fastening two copper wires to the back of the bead using a silver loaded epoxy resin. The assembly was then cast in Quick-Set (R) plastic and the base ground down to expose the alloy. The alloy was polished. The copper wires were then sheathed in Teflon (R) tubing, and the tubing sheath set in Silastic (R) cement where the copper wires entered the anode assembly. The resulting anode assembly was tested as an anode in aqueous sodium chloride. Chlorine bubbles were seen to be evolved and no corrosion was observed.
It is to be understood that although the invention has been described with specific reference to specific details and particular exemplifications and embodiments thereof, it is not to be so limited since changes and alterations therein may be made which are within the full intended scope of this invention as defined by the appended claims.

Number	Name	Date
3293167	Greenspan	Dec 1966
3340097	Hess et al.	Sep 1967
3428544	Bianchi et al.	Feb 1969
3441495	Colman	Apr 1969
3449231	Adams et al.	Jun 1969
3491014	Bianchi et al.	Jan 1970
3852175	Hoekje	Dec 1974

Number	Date	Country
892,720	May 1944	FR
556,083	Sep 1943	UK

Electrolytic anode

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